"The thesis statement is that sentence or two in your text that contains the focus of your essay and tells your reader what the essay is going to be about."
Your thesis should examine the nature of human behavior that is illustrated in this story. A thorough analysis of the motivation of the behavior how the people in this town can continue a tradition, a ritual killing that takes place every year like an annual picnic.
My analysis would include a primitive belief, as exhibited by Old Man Warner, that long held traditions should never be disrespected or disregarded even when it is clear that they have outlived their usefulness. The people in the town can't remember why the lottery was started, or what its purpose is, but it is linked with a bountiful harvest.
There is a suggestion in this story that human sacrifice is necessary for the survival of the population. The needs of the many outweigh the needs of the one, so to speak. The relevance of the human sacrifice has been lost on the modern town, yet they cling to the process, frightened to abandon it, even though it is not wholly supported or highly regarded by the townspeople anymore.